Dr. Wadler was among the doctors, scientists and regulators who emerged in the 1980s and ’90s as sports organizations struggled to keep pace with illicit drug use by athletes. His articulate explanations about steroids and human-growth hormones made him a frequent voice in the news media, at medical conferences, before congressional panels and at trials as an expert witness.

He was equally concerned with medical issues and ethics and how the two intertwined in sports. In 1999, he told the Senate Commerce Committee that performance-enhancing drugs could undermine the Olympic movement and that young athletes could not grow to adulthood believing in “the power of chemical manipulation rather than the power of character.”

To eliminate sports doping, he added, “there can be no compromise, no middle ground, no rhetorical acrobatics.”

Six years later, when Dr. Wadler testified at a House Government Reform Committee hearing about steroid abuse in Major League Baseball, he said that early drug testing by the sport had exposed an unacceptably high number of positive results. He excoriated M.L.B.’s testing policy as one “designed to silence its critics” but not strict enough to detect all possible drug abuse.

“Why should we care?” he asked. “We should care for many reasons, but perhaps most notable is that baseball, our national pastime, for better or worse, is a role-model sport and likely contributes to the alarming abuse of anabolic steroids by teenagers.”

Dr. Wadler was also an expert witness for the Justice Department’s steroid prosecutions and a substance abuse adviser to the National Basketball Association.

Travis Tygart, chief executive of the United States Anti-Doping Agency, said in a statement that Dr. Wadler’s “impact for the good of sport and the health of athletes will long remain.”

Gary Irwin Wadler was born in Brooklyn on Jan. 12, 1939. His father, Samuel, was a window trimmer for stores, and his mother, the former Anne Lowenthal, was a teacher.

After graduating from Brooklyn College and Cornell Medical School, Dr. Wadler started his career as an internist. For many years he was a clinical associate professor of medicine at New York University.

Dr. Wadler acknowledged that he was not very athletic and did not follow sports religiously. He watched sports for relaxation, he said. “I’m the classic two left feet,” he told Newsday in 2003. “I’m still struggling on the tennis court.”

His first professional foray into sports started in 1980, when he became the tournament physician at the United States Open tennis tournament, a position usually held by an orthopedist.

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Dr. Gary Wadler with a patient in his Long Island office, in Manhasset, in 1999.

In 1982, Martina Navratilova, who had not yet won a U.S. Open singles title, visited him and complained of weakness.

“She had swollen glands,” Dr. Wadler told The New York Times. After considering various possibilities, he diagnosed toxoplasmosis when she informed him that she had stayed at a friend’s apartment where there was a cat. Cats can spread the disease.

After winning the first set of her quarterfinal match against Pam Shriver, 6-1, Navratilova physically faltered and lost 7-6 in the second and 6-2 in the third.

“Some people complain of just not feeling well, but I’ve got the number on her,” Dr. Wadler said. “She had to win in two sets. She’s going to feel weak for another month.”

His interest in performance-enhancing drugs was ignited in 1986, during his time at the U.S. Open, when he was asked to take a urine test. (His wife said that he referred to it as a lesson of sorts: “Put your urine where your mouth is.”)

Intrigued, he began to work with Brian Hainline on “Drugs and the Athlete,” a seminal textbook published in 1989. Four years later, the International Olympic Committee honored his work with its President’s Award.

As drug crises swelled in sports during the 1990s and into the 21st century, Dr. Wadler spoke about cases involving the baseball stars Barry Bonds and Mark McGwire, the Olympic sprinter Marion Jones and the basketball player Rashard Lewis.

During the 1998 season, when the Bunyanesque McGwire was on his way to hitting 70 home runs, a single-season record, he admitted to using androstenedione, a dietary supplement. (Years later, he also confessed to using steroids.)

“I don’t think anybody particularly knows whether androstenedione itself has anabolic qualities,” Dr. Wadler told the Times columnist Bob Herbert. “But the substance that androstenedione is converted into unequivocally has anabolic properties. It’s the father of all steroids. Testosterone.”

Dr. Wadler’s expertise brought him to work at the World Anti-Doping Agency, or WADA, where he served as chairman of its prohibited list and methods committee.

In 2011, Dr. Wadler cited the agency’s work when he said that a new National Football League drug-testing program — under which players would be tested for human growth hormone at least once a year — did not go far enough. “If you look at WADA’s antidoping code, the details are incredible,” he told The Times. “It needs to be explicit.”

In addition to his wife, Dr. Wadler is survived by his daughter, Erika; his son, David; two grandchildren; and his brother, Arnold.

After Taylor Hooton, a 17-year-old high school pitcher from Texas, hanged himself in his room in 2003 during withdrawal from steroids, his father, Donald, contacted Dr. Wadler and other experts to try to understand the link between Taylor’s steroid use and his suicide.

Dr. Wadler suggested he create a foundation to educate parents about the dangers of steroids.

A version of this article appears in print on , on Page B13 of the New York edition with the headline: Gary Wadler, Physician Who Became Expert on Doping, Dies at 78. Order Reprints | Today’s Paper | Subscribe